Microporous rubber sheet



Patented Dec. 5, 1939 UNITED STATES MICBDPOROUS RUBBER SHEET Eardley Hazell, New York, N. Y., assignor, by memo assignments, to United States Rubber Company, New York, N. Y., a corporation of New Jersey No Drawing. Application July 5, 1935, Serial No. 30,021

8 Claims.

This invention relates to the production of microporous products from plastic compositions and more particularly to an improved process of producing the same from rubber and rubber-like materials. The invention also relates to the resulting products.

This case is a continuation-in-part of case Serial No. 748,692, filed October 17, 1934, now United States Patent 2,112,529.

10 An object of this invention is to produce microporous articles from thermo-setting plastic compositions. Another object is to produce a microporous rubber body directly from solid rubber (crude or reclaim). A further object is to produce a rubber diaphragm which is adapted to be used as a separator in electric primary or secondary cells, electrolytic cells, filters and the like. A still further object is to produce such a diaphragm which is highly resistant to chemical and electrolytic action, which is highly permeable to water, and which is highly eflicient as'a means for allowing conduction of electric current while preventing passage of line particles through its pores. Other objects will be apparent from the following description.

The invention broadly comprises incorporating in a thermo-setting plastic composition containing lyophilic material, a liquid that is substantially a non-solvent of the plastic and, while maintaining the liquid disseminated throughoutthe mass, setting or curing the composition by heat to a rigid or semi-rigid microporous state under non-evaporative conditions for the absorbed liquid. The mass when set or cured may be of any desired shape. The articles may be embossed, fluted, corrugated or otherwise designed, before, during or after the curing or setting operation.

More specifically, the invention comprises masticating any ordinary commercial grade of solid rubber such as pale crepe, or smoked sheet, or reclaimed rubber, or mixtures thereof, on a mill, with or without the aid of the usual plasticizers, to a smooth plastic consistency and milling into the plastic rubber mass vulcanizing and filling ingredients, together with a relatively large amount of hydrophilic colloid, and water. .The water should be in an amount at least sufficient to permit smooth working of the mass but in- 50 suflicient to invert the rubber phase, that is, such proportions of water should be added as to maintain the whole as an elastic rubber-like mass capable of being sheeted and calendered. Good results are obtainable by using about 1 part of 55 water for every 5 parts of colloid added.

The amount of water will vary somewhat with the nature of the colloid. -It is to be understood that the function of the water in the mixing operation is to facilitate calendering and shaping of the stock and to initiate a swelling of the mix 5 by the attraction between water and its lyophil. The amount of water added in the mixing operation is but part of the water that is necessary to develop the maximum swelling capacity of the rubber mix and therefore additional .water is 10 introduced after the calendering or sheeting operation. The total volume of water introduced into the mass may be of the same order as the volume of the rubber itself in the rubber mix and may evenvexceed somewhat the volume of 5 the rubber.

Where water is the swelling liquid, any hydrophilic colloid may be used, among them being types ordinarily used for making aqueous rubber dispersions, for example, colloidal clays such as 20 bentonite and Silurian shale, starch, water soluble soaps, proteins such as casein, etc. The hydrophil is milled into the rubber, preferably in either dry or aqueous paste form, together with the other desired compounding and vulcanizing 5 ingredients until a uniform mix is obtained, and then this mix has water worked into it gradually, preferably on a mill having corrugated rolls, the addition of the water being eifected in a manner known to those skilled in the art, as to prevent 30 the inversion of the plastic mass to a rubber-inwater dispersion. Accordingly the water is preferably added only about as fast as it is absorbed. This operation may also -be carried out on an open mill with smooth rolls or in a mixer of the closed 35 type such as a Banbury or a Day or a Werner and Pfleiderer machine. It is preferred to use corrugated rolls for the reason that the water is thereby worked into the rubber mass in more intimate association with the hydrophil and that the rubber mix is prevented from slipping and thereby escaping mechanical action. The plastic rubber-like mass thus prepared contains the rubber as the external or continuous phasehaving uniformly dispersed therein the added water in intimate association with the hydrophil. Such a dispersion is sharply distinguished from an aqueous dispersion wherein the water is the external or continuous phase and the rubber the dispersed phase.

After the addition of the water on the mill or mixer, the rubber mass is ready for shaping, as by calendering, which may be at once or after allowing the mass to stand awhile. The mass is cal- 'endered or otherwise sheeted to a desired thickness, or may be processed in any other manner into the shape of the article desired, as by extruding, molding, etc. If desired, the sheeted or otherwise shaped rubber mass may be submerged in water to allow imbibition of more water. A fair degree of swelling results if the compound is soaked in water at room temperature for a period of at least 24 hours. If higher swelling temperatures are used, they should be below the temperature at which the mass sets or cures.

When the material has been sheeted it may be wrapped on a mandrel with a wet liner for the material. This may be done by rolling the stock from the calender into a wet liner, and then rerolling onto a mandrel between a wet liner after passage through a water bath. In order to allow the sheet to swell freely without too much confining pressure and to give a product of uniform gauge, the longitudinal edges of the liner may be built up by a fabric tape of. suitable thickness to provide a liner having raised edges or borders. Alternatively the sheeted stock may be wrapped in a single or double nappedsheeting, preferably of coarse weave, thus providing a cushioning effect against pressures developed during swelling. The wrapping tension should not be greater than that necessary to avoid wrinkling of the liner so as not to interfere with proper swelling and uniform gauge.

After soaking the stock in water at room temperature or at a higher temperature below the vulcanizing temperature, the stock is given a cure under non-evaporative conditions for the absorbed water. as by vulcanizing the shaped rubber mass, preferably wrapped as described, while it is under water heated to a vulcanizing temperature. Such a cure is referred to as a submarine cure. The resulting vulcanizate is permeated with minute pores which may range from an average pore diameter of 0.5 micron or less to about 6.5 microns. The larger size is more desirable for filters. Smaller average diameters may be obtained by tighter wrapping of the stock during the swelling and/pr curing stages, and by regulation of loading with lyophilic colloid. The increase in gauge of. the stock as a result of the imbibition of water is usually of the order of 100 to 150%.

After curing, the articles may be dried, but are preferably kept wet to exclude air, for example by submergence in water. The microporous structure remains intact and is highly permeable to liquids such as electrolyte solutions.

If better dispersion of the lyophil and better distribution of the liquid is desired, a peptizing agent for the lyophil, as distinguished from a fiocculating agent, may be added with the lyophilic colloid. The use of the peptizing agent increases the average pore diameter of the product, without appreciably changing the percentage of total voids. This favors lower electrical resistance to electrolyte solutions.

If desired, finely divided cellulosic material ma be added to the mix and/or various wetting agents may be added, in order to modify the physical and electrical properties of the product, as desired.

In the manufacture of electric battery plate separators, sheets of the highly hydrated rubber composition of the desired thickness and containing a proportion of sulphur suitable for the production of hard rubber, are preferably wrapped on a mandrel and vulcanized under water as above described, and are then rolled out flatand out to the desired final dimensions while still hot and wet. If desired, they may be fluted or corrugated, as by passing them between suitable corrugated calender or other rolls or by pressing them in suitable molds, while hot. Alternatively, the dried separators may be ribbed as by cementing thereon ribs of porous hard rubber, hard rubber, wood, etc., by means of a cement and then may be placed in water and subjected to a vacuum for at least an hour, or boiled in water, to displace air from the separator. As another alternative, sheets of suitable thickness may be calendered' and cured as above described, and may then be ground down to form channels in one face, leaving reinforcing bands, ridges, or ribs between the channels. The separators should afterwards be kept-wet, as by immersion under water until assembled in a battery.

The following example is given to illustrate the invention more particularly in the production of battery plate separators. The parts are by weight:

ExampZe.--A mix is prepared from the following formula:

The example refers to about 145 parts of vulcanized hard rubber on the basis of a dry mix of about 291.2 parts or about 49.8% of hard rubber, even when the mix contains the optional ingredients. The proportion of hard rubber rises above 50% where the optional ingredients are reduced or not employed or where the proportion of non-rubber ingredients is reduced by solution as by the treatment of the cured plate with alkali, acid and/or water.

Nekal is a wetting agent and is understood to be sodium isopropyl naphthalene sulphonate. It renders the separator capable of being rapidly wetted and permeated by water or electrolyte solution upon contact therewith, and hence serves to reduce the initial internal electrical resistance upon immersion of the separator in battery acid to a small fraction of the initial resistance where no wetting agent is used, and to promote rapid at.- tainment of the equilibrium value of the internal resistance. Other wetting agents may be used instead. The sodium silicate peptizes the bentonite and increases the average pore diameter of the finished plate. Instead of diphenylguanidine other accelerators of vulcanization may be used. As finely divided cellulosic materials, alpha cellulose, beta cellulose, and ordinary wood flour are satisfactory examples. Alpha cellulose is less resistant to the hydrolyzing action of acids and alkalies than beta cellulose. The cellulosic material further reduces the internal electricalresistance of the separator and especially so when the cured separator containing the cellulose has been submitted to the action of a hydrolyzing or a swelling agent for the cellulose, for example sulphuric acid of 30-50% weight concentration, or 1 to 10% sodium hydroxide solution, at elevated temperatures, for example 100-120 C. After the acid or alkali treatment, the separators are washed with water and. kept wet until used. It is believed that in the case of acid treatment, partial hydrolysis of the cellulose confers on the cellulose a gel structure similar to that resulting from the action of battery acid on wood separators. The sulfuric acid treatment appears also to produce wetting agents in situ by reaction with some constituents of the cellulosic material as well as inherently removing some acid solubles from the clay such as iron, etc. Accordingly the addition of a wetting agent as such, such as, Nekal, may be dispensed with when the sulfuric acid treatment is employed. The acid treatment may be carried out before or after the stock is cut to size.

In preparing each mix the rubber is first well broken down, and all the ingredients, except the water, then added and dispersed uniformly throughout the rubber in known manner while cooling the mill rolls. The batch is then transferred to a corrugated roll mill and the water added as rapidly as it is absorbed by the mass. The mix is then warmed up, calendered to about 40 percent of the ultimately desired thickness. taken up on a water-soaked fabric liner, the edges of which are taped as described, and the calendered sheet with the liner is wrapped on an aluminum mandrel under sufficient tens on to prevent wrinkling. The wrapped stock may be made to absorb still more water by immersing it in warm water for several hours, and this procedure is desirable when the amount of water introduced by milling has been less than parts per 100 parts of rubber. At room temperature the immersion should be for at least 12 hours, and may be supplemented by preheating the compound in the vulcanizer under water at a temperature of about C. for at least one hour in order to develop maximum swelling. The material is then vulcanized to hard rubber by bringing the water rapidly to the curing temperature for example 156 C. and maintaining the curing temperature for about 3 hours. Another schedule is, for example, heating one hour under water at from to C. followed by five hours at about C. The temperature is brought down gradually with cold water before releasing the pressure in the vulcanizer. During the course of the cure there is a substantial increase in the gauge of the sheet. The hot wet stock may then be cut to the desired size.

The microporous battery plate separators prepared by the described process are found to have an average pore diameter of from 0.5 to 4 microns. The percentage of voids ranges from 55 to 65% and is dependent on the amount of water imbibed into the rubber composition before cure, which in turn may be controlled by (a) the loading of lyophil colloid, (b) the time and temperature of the soaking in water, and (c) the tightness of wrapping the calendered sheet.

A wide variation is permitted in the proportion of lyophil colloid employed, the weight thereofv preferably not exceeding the weight of the rubber, in order to avoid undue brittleness in the vulcanized product. In the'case of whole latex rubber such as spray dried latex rubber, which contains a small amount of natural hydrophilic materials, the amount of added hydrophil need not be as great as with other rubbers lacking hydrophils. Likewise the precise amount of water to be incorporated in the mass by milling, is immaterial, since further quantities of water are readily absorbed by immersion of the mass in wat 1r. It is probable that still further water is absorbed during the submarine cure.

An efficient battery plate separator should have as high a percentage of pore voids as is consistent with fair mechanical strength, and the upper limit of pore diameter should be such as to prevent lead tree growth in the battery. Furthermore the separator should be of suflicient strength, even with a fairly high percentage of voids, to withstand handling without breakage.

The separator of the present invention is distinguished from other known hard rubber sepa rators in the following respects:

(a) In strength it is superior to the compacted hard rubber dust separators and to separators produced directly from latex. The rubber dust separators are even too fragile to cut for test samples. The latex separators approximate a strength of 500-600 pounds per sq. in. in all directions, whereas a separator can be prepared by the present invention which approximates a strength of from about 1000 to about 1200 lbs. per sq. inch in the direction of the grain (due to the calendering) and approximately 500 to 600. pounds per sq. inch across the grain.

(b) The percentage of voids in applicants product ranges as high as 65% whereas the maximum percentage found in the latex separators is about 53%, and in the rubber dust separators about 50%. In the case of ribbed separators, ribbed on one side, applicants products show a percentage of total voids of at least about 60%, figuring 50 to 60% total voids for the ribs, and 55 to 65% total voids for the thin intervening webs which make up on the order of 80% of the area of the plates.

(0) The separator of the present invention has a lower equilibrium resistance than the other separators. By equilibrium resistance is meant the constant electrical resistance that the separator exhibits after the battery is put in operation and currenthas been drawn from it long enough for the resistance of the separator to reach a constant minimum value. Wood separators require several days before they reach this value, while the separators of this invention, made from a rubber composition containing a wetting agent as above described, reach a constant value in about one-half hour. The resistance curve drops from the time the current is turned on and adjusts itself to a substantially constant minimum value. For example, at 0 F. a separator of the present invention registered around 20 percent lower equilibrium resistance than the other products. This is a desirable property because it will allow an electric battery to more quickly start a motor car engine at low temperatures. Even at temperatures as high as 80 F. the separator of the present invention has an equilibrium resistance around '5 percent lower than that of the other types of separators.

The term rubber is to be construed broadly as including caoutchouc, gutta percha, balata, rubber isomers, as well as other rubber-like substances which similarly to rubber may be vulcanized or set.

Other rubbers that may be treated within the broad scope of the invention are the evaporation residues of the various latices, for example spray dried whole latex rubbers. In addition, the invention includes within its broad scope variations 'such as adding a hydropbile, that is. a material which is capable of absorbing and of being swelled by water, to latex or aqueous dispersion of rubber, drying and shaping the mass as tiesired, soaking the shaped mass in an aqueous medium and then curing the mass under nonevaporative condition for the water.

It will be apparent to-those skilled in the art that certain modifications in the amounts of materials specified in the examples may be made, and that impure or commercial grades of materials may be used in place of chemically pure substances, without departing from the principles of the invention as covered by the appended claims.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. A vulcanized hard rubber microporous sheet composed essentially of a continuous matrix of masticated solid crude rubber vulcanized to the hard rubber state and permeated with pores of microscopic size only, the vulcanized hard rubber constituting at least about one-half of the sheet by weight, and said rubber having the longitudinal grain characteristic of calendered rubber.

2. A vulcanized hard rubber microporous sheet composed essentially of a continuous matrix of masticated solid crude rubber vulcanized to the hard rubber state and permeated with pores of microscopic size only, the vulcanized hard rubber constituting at least about one-halfofthesheetby weight, said rubber having the longitudinal grain characteristic of calendered rubber, said vulcanized rubber being intimately associated with particles of inorganic colloid.

3. A vulcanized hard rubber microporous sheet composed essentially of a continuous matrix of masticated solid crude rubber vulcanized to the hard rubber state and permeated with pores of microscopic size only, the vulcanized hard rubber constituting at least about one-half of the sheet by weight, said rubber having the longitudinal grain characteristic of calendered rubber, and

having at least 55% of total voids.

4. A microporous vulcanized hard rubber battery plate separator composed essentially of a continuous matrix of masticated solid crude rubber vulcanized to the hard rubber state and permeated with pores of microscopic size only, the vulcanized hard rubber constituting at least about one-half of the separator by weight, said rubber having the longitudinal grain characteristic of calendered rubber, and said rubber being associated with sulphuric acid-insoluble ingredients of a hydrophilic colloidal clay and with finely divided cellulosic material.

5. A mlcroporous vulcanized hard rubber battery plate separator composed essentially of a continuous matrix of masticated solid crude rubber vulcanized to the hard rubber'state and permeated with pores of microscopic size only, the vulcanized hard rubber constituting at least about one-half of the separator by weight, said rubber having the longitudinal grain characteristic of calendered rubber, and said rubber being associated with sulphuric acid-insoluble ingredients of a hydrophilic colloidal clay and with finely divided cellulosic material, and which separator is further characterized in having at least 55% total voids and a low equilibrium electrical resistance.

6. A microporous vulcanized hard rubber highly water-permeable battery plate separator composed essentially of a continuous matrix of masticated solid crude rubber vulcanized to the hard rubber state and permeated with pores of average effective diameter not exceeding four microns, the vulcanized rubber constituting at least about one-half of the separator by weight, and said rubber having the longitudinal grain characteristic of calendered rubber.

'7. A microporous vulcanized hard rubber highly water-permeable battery plate separator com-. posed essentially of a continuous matrix 01' masticated solid crude rubber vulcanized to the hard rubber state and permeated with pores of average eifective diameter not exceeding four microns, the vulcanized rubber constituting at least about one-half of the separator by weight, said rubber having the longitudinal grain characteristic of calendered rubber, said rubber being intimately associated with particles of inorganic colloid.

8. A microporous vulcanized hard rubber battery plate separator having the longitudinal grain characteristic of calendered rubber, and in which the rubber phase is continuous and constitutes at least about one-half of the separator, by

weight, said rubber being intimately associated with dispersed finely divided cellulosic material, and colloidal clay ingredients in a proportion by weight'not exceeding the weight of the hard rubber. 1

EARDLEY HAZELL. 

